Injection molding is a technology commonly used for high-volume manufacturing of parts made of meltable material, most commonly of parts made of thermoplastic polymers. During a repetitive injection molding process, a plastic resin, most often in the form of small beads or pellets, is introduced to an injection molding machine that melts the resin beads under heat, pressure, and shear. The now molten resin is forcefully injected into a mold cavity having a particular cavity shape. The injected plastic is held under pressure in the mold cavity, cooled, and then removed as a solidified part having a shape that essentially duplicates the cavity shape of the mold. The mold itself may have a single cavity or multiple cavities. Each cavity may be connected to a flow channel by a gate, which directs the flow of the molten resin into the cavity. Thus, a typical injection molding procedure comprises four basic operations: (1) heating the plastic in the injection molding machine to allow it to flow under pressure; (2) injecting the melted plastic into a mold cavity or cavities defined between two mold halves that have been closed; (3) allowing the plastic to cool and harden in the cavity or cavities while under pressure; and (4) opening the mold halves to allow the part to be ejected from the mold.
The molten plastic resin is injected into the mold cavity and the plastic resin is forcibly pushed through the cavity by the injection molding machine until the plastic resin reaches the location in the cavity furthest from the gate. The resulting length and wall thickness of the part is a result of the shape of the mold cavity.
The molds used in injection molding machines must be capable of withstanding these high melt pressures. Moreover, the material forming the mold must have a fatigue limit that can withstand the maximum cyclic stress for the total number of cycles a mold is expected to run over the course of its lifetime. As a result, mold manufacturers typically form the mold from materials having high hardness, such as tool steels, having greater than 30 Rc, and more often greater than 50 Rc. These high hardness materials are durable and equipped to withstand the high clamping pressures required to keep mold components pressed against one another during the plastic injection process. Additionally, these high hardness materials are better able to resist wear from the repeated contact between molding surfaces and polymer flow.
High production injection molding machines (i.e., class 101 and class 102 molding machines) that produce thinwalled consumer products exclusively use molds having a majority of the mold made from the high hardness materials. High production injection molding machines typically produce 500,000 parts or more. Industrial quality production molds must be designed to produce at least 500,000 parts, preferably more than 1,000,000 parts, more preferably more than 5,000,000 parts, and even more preferably more than 10,000,000 parts. These high production injection molding machines have multi cavity molds and complex cooling systems to increase production rates. The high hardness materials described above are more capable of withstanding the repeated high pressure clamping and injection operations than lower hardness materials. However, high hardness materials, such as most tool steels, have relatively low thermal conductivities, generally less than 20 BTU/HR FT ° F., which leads to long cooling times as heat is transferred from the molten plastic material through the high hardness material to a cooling fluid.
In an effort to reduce cycle times, typical high production injection molding machines having molds made of high hardness materials include relatively complex internal cooling systems that circulate cooling fluid within the mold. These cooling systems accelerate cooling of the molded parts, thus allowing the machine to complete more cycles in a given amount of time, which increases production rates and thus the total amount of molded parts produced. However, these cooling systems add complexity and cost to the injection molds. In some class 101 molds more than 1 or 2 million parts may be produced, these molds are sometimes referred to as “ultra high productivity molds” Class 101 molds that run in 400 ton or larger presses are sometimes referred to as “400 class” molds within the industry.
High hardness materials are generally fairly difficult to machine. As a result, known high throughput injection molds require extensive machining time and expensive machining equipment to form, and expensive and time consuming post-machining steps to relieve stresses and optimize material hardness. Milling and/or forming cooling channels within these complex molds adds even more time and costs to the manufacture of typical high throughput injection molds.
There is a tradeoff between machining complexity and cooling efficiency in traditional, high hardness molds. Ideally, cooling channels should be machined as close to the mold cavity surfaces as possible. Additionally, conformal cooling is desirable and most effective. However, machining conformal cooling channels close to molding surfaces is difficult, time consuming, and expensive. Generally, machining cooling channels within about 5 mm of the mold surfaces is considered to be the practical limit. This practical limit reduces cooling efficiency due to material between the cooling fluid and the hot plastic having low thermal conductivity. Conventional machining techniques, along with conventional mold materials (i.e., high hardness and low thermal conductivity) place a lower limit on cycle time and cooling efficiency for a given mold.
Furthermore, locating cooling lines close to the mold surfaces requires precise machining of the cooling lines in the molds. Because the molds are supported by mold support plates when placed in a clamping device of the injection molding machine, fluid seals must be located where the cooling lines transition from the mold support plate to the mold (because the fluid circulating systems (e.g., pumps) must be located outside of the molds). These fluid seals may fail, causing cooling fluid to escape. As a result, parts may be incompletely cooled, which produces an inferior part, or the plastic in the mold may be contaminated with cooling fluid, which is also undesirable.
Still further, practical limitations on machining cooling channels results in unequal cooling within the mold. As a result, temperature gradients are produced within the mold cavity. Often the temperature of the surface of a mold cavity can vary by ten degrees Celsius or more. This wide variation in temperature within the mold can lead to imperfections in the molded parts.